Substrates including useful layers

ABSTRACT

Substrates may include a useful layer affixed to a support substrate. A surface of the useful layer located on a side of the useful layer opposite the support substrate may include a first region and a second region. The first region may have a first surface roughness, may be located proximate to a geometric center of the surface, and may occupy a majority of an area of the surface. The second region may have a second, higher surface roughness, may be located proximate to a periphery of the surface, and may occupy a minority of the area of the surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/324,461, filed Feb. 8, 2019, now U.S. Pat. No. 10,950,491, issuedMar. 16, 2021, which is a national phase entry under 35 U.S.C. § 371 ofInternational Patent Application PCT/FR2017/052161, filed Aug. 1, 2017,designating the United States of America and published as InternationalPatent Publication WO 2018/029419 A1 on Feb. 15, 2018, which claims thebenefit under Article 8 of the Patent Cooperation Treaty to FrenchPatent Application Serial No. 1657722, filed Aug. 11, 2016.

TECHNICAL FIELD

The present disclosure relates to a method for transferring a usefullayer.

BACKGROUND

A method for transferring a useful layer 3 onto a support substrate 4 isknown from document FR3020175, as shown in FIG. 1 , with such methodincluding the following main steps:

-   -   a) forming an embrittlement plane 2 by implanting low atomic        weight elements into a first substrate 1, so as to form a useful        layer 3 between such plane and one surface of the first        substrate 1;    -   b) applying the support 4 onto the surface of the first        substrate 1 so as to form an assembly to be fractured 5 having        two exposed surfaces S1, S2;    -   c) applying a heat treatment for embrittling the assembly to be        fractured 5;    -   d) initiating and propagating a fracture wave into the first        substrate 1 along the embrittlement plane 2.

According to this document, acoustic vibrations are emitted upon theinitiation and/or the propagation of the fracture wave. The interactionbetween the fracture wave and such acoustic vibrations results in theforming of periodical patterns of variation in the thickness of theformed useful layer, which extend on the whole surface of the layer. Inother words, the fracture wave is vertically deviated relative to theprogression plane thereof, according to the state of the instantaneousstress of the material it goes through, with such stress state beingaffected by the acoustic wave. FIG. 2 thus shows the patterns ofvariation in the thickness of the useful layer transferred according tothe method disclosed above. In the illustrated example, the patternscomprise side and circular patterns (respectively indicated by a and bin FIG. 2 ). Such patterns have been made apparent by measuring the“haze” as per the current English terminology, corresponding to theintensity of light broadcast by the layer surface, using the inspectiontool SURFSCAN™ by the KLA-Tencor company. Reference can be made to thearticle “Seeing Through the Haze” by F. Holsteyns, Yield ManagementSolution, Spring 2004, pp. 50-54 for further information on suchinspection technique.

To solve this problem, the above document teaches equipping the assemblyto be fractured with an absorbing element for detecting and dissipatingthe emitted acoustic wave, and preventing or limiting the formation ofsuch patterns of variation in the thickness of the useful layer.Although this method is efficient, it nevertheless requires theimplementation of an absorbing element on at least one of the faces ofthe assembly to be fractured, which makes the method for transferringthe useful layer more complex.

BRIEF SUMMARY

One object of the present disclosure is to provide a simple method fortransferring a useful layer, with such useful layer having a reducedpattern of variation in the thickness thereof.

To this end, the disclosure provides a method for transferring a usefullayer onto a support comprising the following steps:

-   -   forming an embrittlement plane by implanting light species into        a first substrate, so as to form a useful layer between the        plane and one surface of the first substrate;    -   applying the support onto the surface of the first substrate so        as to form an assembly to be fractured;    -   applying a heat treatment for embrittling the assembly to be        fractured;    -   initiating and propagating a fracture wave into the first        substrate along the embrittlement plane.

The method is characterized in that the fracture wave is initiated in acentral area of the embrittlement plane and the propagation speed of thewave is controlled so that the velocity thereof is sufficient. Thus, theinteractions of the fracture wave with acoustic vibrations emitted uponthe initiation and/or propagation thereof are limited to a peripheralarea of the useful layer 3.

The method disclosed herein makes it possible to limit the formation ofperiodical patterns of variation in the thickness to one part only ofthe useful layer.

According to other advantageous and not restrictive characteristics ofthe disclosure, taken either separately or in any technically feasiblecombination:

-   -   the propagation speed of the fracture wave is controlled so as        to have a velocity higher than one third of the speed of the        acoustic wave;    -   the first substrate is made of silicon, and the propagation of        the fracture wave is controlled so as to have a velocity above 2        km/second;    -   the propagation of the fracture wave is controlled so as to have        a velocity ranging from 2 km/second to 4.5 km/second; preferably        from 3.8 km/second to 4.2 km/second;    -   the fracture wave is initiated by applying the embrittlement        heat treatment and the propagation speed is controlled by        selecting the temperature of the heat treatment upon the        initiation;    -   the temperature of the embrittlement heat treatment is above        400° C., or 500° C.;    -   the initiation of the fracture wave is caused by an incipient        fracture positioned in the central area of the embrittlement        plane, or in the vicinity of the central area;    -   the incipient fracture consists of a volume containing low        atomic weight elemental species at a higher concentration        compared to the average concentration in the embrittlement        plane;    -   the incipient fracture consists of a cavity or a body positioned        at the interface between the first substrate and the support in        line with the central area of the embrittlement plane;    -   the fracture wave is initiated by energy input at the central        area of the embrittlement plane;    -   the propagation speed is controlled by calibrating the        embrittlement heat treatment so that the embrittlement plane has        a maturing rate at least equal to a target maturing rate;    -   the first substrate consists of a disk-shaped wafer, and the        central area comprises the geometric center of the wafer;    -   the peripheral area consists of an annular area, the inner        radius of circle of which is greater than ⅔, and preferably        greater than 80%, of the radius of the first substrate;    -   the peripheral area is totally encompassed in one exclusion area        of the useful layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the disclosure will be clearfrom the following detailed description, made in reference to theaccompanying figures, among which:

FIG. 1 shows a method for transferring a useful layer according to thestate of the art;

FIG. 2 shows a view of the haze emitted by a useful layer of the priorart showing the periodical patterns of variation in the thickness;

FIG. 3 shows a diagram of the experimental relationship between thespeed of the fracture wave and the speed of the acoustic wave;

FIG. 4 shows a method for transferring a useful layer according to thedisclosure;

FIG. 5A shows a view of the haze emitted by a useful layer obtained by amethod according to the disclosure;

FIG. 5B shows a view of a useful layer obtained by a simulation of amethod according to the disclosure.

DETAILED DESCRIPTION

The phenomena disclosed in the presentation of the prior art has beenthoroughly studied to provide an original method for transferring auseful layer. This original approach does not aim at limiting thedevelopment of the acoustic wave, for instance, by collecting it and/orby absorbing it, but at limiting the interaction thereof with thefracture wave. The forming of the periodical patterns of variation inthe thickness is thus limited to one part only of the layer.

Thus, it was noted that the acoustic wave would propagate from afracture initiation point, at a speed higher than that of the fracturewave, and that the origin of the wave pattern originated from theinterferences caused by the collision between the fracture wave and thereflection, on the ends of the assembly to be fractured, of the acousticwave. The propagation characteristics of the acoustic wave, andspecifically the speed thereof, can be measured by piezoelectric sensorspositioned on one and/or the other faces of the assembly to befractured.

To limit such interferences, the method disclosed herein provides, onthe one hand, placing the point of initiation of the fractures away fromthe ends of the assembly to be fractured, i.e., to placing the point ofinitiation at a distance from the substrate edges, at a central area ofthe embrittlement plane. Thus, the moment when the reflected acousticwave meets, the fracture wave is delayed relative to the fractureinitiation time.

Simultaneously, the method disclosed herein involves controlling thespeed of propagation of the fracture wave. The speed of propagation ofthe fracture wave is selected to be high enough for the area where thefracture wave and the reflected acoustic wave (which will have theperiodical patterns of the variation in thickness of the useful layer)to be confined to a peripheral area of the assembly to be fractured. Inthe most advantageous situations, specifically if the speed of thefracture wave is high enough, the peripheral area can be encompassed inone exclusion area of the useful layer, i.e., an area where componentsare generally not intended to be formed. Such exclusion area may be 0.5to 2 or 3 millimeters, at the periphery of the useful layer. The surfaceof the useful layer not encompassed within the peripheral area is mainlyfree of periodical patterns and thus has the required characteristics asregards thickness uniformity.

It should be noted that the principle of the disclosure is based on theassumption, verified by the inventors of the present application, thatthe speed of propagation of the acoustic wave has no direct (forinstance, proportional) link with the speed of propagation of thefracture wave.

As a matter of fact, if an assembly to be fractured having a circularshape and a radius R, is considered, Vg indicates the speed of theacoustic wave in the plane defined by the assembly to be fractured, andVf is the speed of the fracture wave, then the distance d between thecenter of the assembly to be fractured (from where the fracture isinitiated) and the place where both waves collide meets the followingequation:d/Vf=(2R−d)/Vgassuming that, at the meeting point, the fracture wave has travelled ona distance d, at a speed Vf, and the reflected acoustic wave hastravelled on a distance (2R−d) at a speed Vg. It can thus be determinedthat such distance d satisfies the following equation:d=2R/1+Vg/Vf

It can thus be noted that such distance d can be controlled so as to beas close to R as possible, insofar as the Vg/Vf ratio can be controlledtoo, to be close to 1.

FIG. 3 shows results of experiments carried out by the inventors of thepresent application, showing the Vg/Vf ratio for various speeds Vf ofthe fracture wave. Such measurements have been obtained when the methodfor transferring a layer is applied to a first silicon substrate.

It can be noted that the Vg/Vf ratio can be selected from a wide rangefrom 1 to 2, by selecting the value of the fracture wave speed Vf. For alow speed of the fracture wave, of the order of 1 km/second, the speedof the acoustic wave is almost twice as high (Vg/Vf close to 2).Therefore, the meeting area is an annular area, the inner radius ofwhich is equal to ⅔ of the radius R of the assembly to be fractured.

For a high speed of the fracture wave, the speed of the acoustic wave isslightly higher than that of the fracture wave (Vg/VF close to 1). Themeeting area is then limited to an annular area, the inner radius ofwhich is very close to the radius R of the assembly to be fractured, andso almost non-existent.

On the basis of the study, the present disclosure provides a method fortransferring a useful layer 3 onto a support 4 comprising the steps asshown in FIG. 4 , Panels a) to d).

In a first step as shown in FIG. 4 , Panel a), an embrittlement plane 2is formed, for instance, by implanting low atomic weight elementalspecies into the first substrate 1. The first substrate 1 may be made ofsilicon, or any other material, whether semi-conductive or not. It maybe germanium, for instance, or gallium nitride, lithium tantalate, orsapphire. Layers can be deposited beforehand, onto the surface, using adeposition or a heat treatment. It may be a layer made of silicondioxide, obtained by thermal oxidation of a silicon board or a layerobtained by any kind of epitaxial deposition: SiGe; InGaN; AlGaN, Ge,etc.

The first substrate 1 may be a disk-shaped wafer, with standarddimensions, for instance, 200 mm or 300 mm, or even 450 mm in diameter.However, the disclosure is in no way restricted to these dimensions orthis shape.

As for the low atomic weight elemental species, these may be any speciesable to embrittle the first substrate 1 at the embrittlement planethereof. It may be hydrogen and/or helium.

The embrittlement plane 2 defines, with a surface of the firstsubstrate, the useful layer 3.

In a second step shown in Panel b), a support 4 is applied onto thesurface of the first substrate 1 so as to form an assembly to befractured 5. The support 4 may comprise a silicon wafer, or a substratemade of any material and having any shape, for instance, made ofsapphire or glass. Like the first substrate 1, the support 4 may beprovided, beforehand, with superficial layers of any kind.

The support 4 can be applied onto the surface of the first substrateusing any direct assembling method: molecular adhesion,thermo-compression, electrostatic bonding, etc. It may also compriseapplying an adhesive layer onto the support 4 and/or the first substrate1, such as glue. The application of the support 4 may also involve thedeposition of a material onto the surface of the first substrate 1, withthe deposited layer forming the support 4.

In a further step shown in Panel c), an embrittlement heat treatment isthen applied to the assembly to be fractured 5. Such a heat treatmentweakens the first substrate 1 at the embrittlement plane 2, and suppliessufficient energy for the fracture wave, once initiated, to propagate ina self-sustained way. Such a heat treatment is typically carried out attemperatures ranging from 150° C. to 600° C. for 30 minutes to 8 hours,for instance, for 4 hours at 400° C.

In a first alternative embodiment, the heat treatment is sufficient, initself, to initiate the fracture wave. Upon completion of the heattreatment, the useful layer 3 is totally removed from the firstsubstrate 1, as shown in FIG. 4 , Panel d).

In a second alternative embodiment, shown in FIG. 4 , Panel c′), themethod comprises a local addition of energy, prior to, or during theembrittlement heat treatment, to initiate the fracture wave. Such energymay have a mechanical, thermal, or any other origin. It may originatefor a local heating by a laser, for instance, or ultrasonic energyinput.

Whatever the alternative embodiment, once initiated, the fracture wavepropagates in a self-sustained way so as to detach and transfer theuseful layer 3 onto the support 4, as shown in Panel d) in FIG. 4 .

According to the present disclosure, the fracture wave that is createdduring one of the steps shown in FIG. 4 , Panels c) and c′) is initiatedat a central area of the embrittlement plane 2. In these figures, theinitiation in the central area is symbolized by large arrows.

The fracture wave does not have to be initiated precisely at thegeometric center of such embrittlement plane 2. Thus, the central areaof initiation may correspond to a circular surface area substantiallycentered on the geometric center of the embrittlement plane. Such asurface area may correspond to 20%, 10% or 5% of the total surface ofthe embrittlement plane 2. As explained above, when the fracture isinitiated at one point of this area, it is far enough from the edges ofthe assembly to be fractured 5 to delay the moment when the fracturewave interacts with the reflected acoustic wave.

Several approaches are possible to cause the initiation of the fracturein such a central area. An incipient fracture can thus be placed in thisarea or close thereto, in a step prior to the step of heat treatment.During this step, the incipient fracture is an advantageous place forinitiating such fracture.

Such an incipient fracture can thus be formed by introducing low atomicweight elemental species into the first substrate 1, at the central areaof the embrittlement plane 2. A volume, which defines the incipientfracture, is thus formed, and has light elemental species at a higherconcentration relative to the average concentration of the embrittlementplane 2. Such excess light elemental species can thus be introducedprior to, during, or after the step of forming the embrittlement plane2, and in any case, prior to the step of assembling the support 4 andthe first substrate 1. The main dimensions of the incipient fracture canrange from 10 nm to a few millimeters. The excess elemental species canbe introduced into the first substrate 1, by means of a localimplantation, with or without a previous masking of the first substrate1. For instance, a local implantation of hydrogen on a surface area, 1mm in diameter, in a dose (in addition to the one forming theembrittlement plane) of 1e¹⁶ at/cm² makes it possible to initiate thefracture during the step of embrittlement heat treatment, specificallyat such over-implanted area forming the incipient fracture.

The incipient fracture can also be formed at the assembling interfacebetween the first substrate 1 and the support 4, in line with thecentral area of the embrittlement plane 2. It may comprise a cavity, forinstance, made by etching, at the surface of the first substrate 1 orthe support 4, or of a body having reduced dimensions positioned at suchinterface. The presence of such body or cavity results in local stressduring the embrittlement heat treatment, which favors the initiation ofthe fracture wave in the central area, as disclosed, for instance, indocument US 2010/330779.

When the fracture wave is caused during the step of embrittlement heattreatment because of the presence of an incipient fracture, the heattreatment can be evenly applied to the assembly to be fractured 5.

As an alternative to the insertion of an incipient fracture, or inaddition thereto, the initiation can be caused by local energy input atthe central area during or after the step of the embrittlement heattreatment. The heat treatment equipment may be configured so that,during the treatment, the central area receives more heat energy thanthe peripheral area.

In some embodiments, the local application of energy may be appliedusing a light beam (a laser, for instance) or a particle beam (ions,electrons).

In some embodiments, additional mechanical energy may be applied suchas, for example, vibrations transmitted by an ultrasonic generator, forinstance, a piezoelectric generator, at the central area.

In the last two examples, local energy input can be carried out duringthe step of the embrittlement heat treatment or during a possibleadditional step dedicated to such local energy input.

Whatever the technique selected to initiate the fracture at the centralarea of the embrittlement plane 2, the present disclosure also comprisescontrolling the fracture wave propagation speed so that it has asufficient speed, thus limiting the interactions with the acousticvibrations emitted upon the initiation and/or propagation of thefracture wave.

As explained above, a sufficient speed makes it possible to limit suchinteractions so that they occur, if at all, only within a peripheralarea. The higher the speed of the fracture wave, the smaller the surfacearea of the peripheral area in which the interactions may occur. Theperiodical patterns of variation in thickness are thus limited to thisperipheral area of the useful layer 3. Thus, when the first substrate ismade of silicon, the fracture wave propagation can advantageously becontrolled to have a speed above 2 km/second and preferably between 2and 4.5 km/second, or more preferably equal or close to 4 km/second, forexample from 3.8 to 4.2 km/second.

More generally, and according to one preferred embodiment of thedisclosure, the fracture wave propagation speed will be controlled sothat it is higher than or equal to one third of the acoustic wave speed(i.e., Vg/3), so that the peripheral area, wherein the periodicalpatterns are confined, is smaller than R/2.

Depending on the method selected for initiating the fracture (whetherthermal and/or mechanical), the operational parameters of such methodcan be so selected as to control the fracture wave propagation speed.For this purpose, use can be made of the device for measuring thefracture wave speed disclosed in document WO 2013/140065.

Thus, when the fracture initiation is obtained, as regards temperature,for instance, when the embrittlement heat treatment causes suchinitiation by itself, reference can be made to the document “FractureDynamics in Implanted Silicon” by D. Massy et al., Applied PhysicsLetters 107 (2015), to select the temperature making it possible toobtain the targeted fracture wave speed. Particularly, a temperatureabove 400° C. or 500° C. can be selected. The document discloses amethod making it possible to link the fracture wave propagation speed tothe wave embrittlement and/or initiation operational parameters. Itspecifically mentions that, in the case of a first silicon substrate,the fracture wave propagation speed can be controlled betweenapproximately 1 km/second and 4 km/second when the temperature, upon theinitiation of the fracture, varies from about 300° C. to 700° C.

In embodiments that comprise a local energy input to initiate thefracture wave, the maturing rate of the embrittlement plane, as obtainedupon completion of the embrittlement heat treatment, can vary. Thematuring rate corresponds to the surface covered by micro-cracks formedin the embrittlement plane, upon applying local energy, causing theinitiation of the fracture. As a matter of fact, the fracture wavepropagation speed depends on the maturing rate: the higher theparameter, the higher the fracture wave propagation speed.

Maturing rates can be measured by measuring the surface covered by themicro-cracks, for instance, using an infrared microscope to calibratethe embrittlement heat treatment making it possible to reach a targetfracture speed. Such heat treatment can also be adjusted so that theembrittlement area has a maturing rate at least equal to a targetmaturing rate, which makes it possible to reach or exceed a targetedfracture speed.

FIG. 5A shows a view of the haze emitted by a useful layer obtained by amethod according to the present disclosure.

The useful layer 3 of FIG. 5A results from a method comprising a step offorming an incipient fracture by implanting an amount of 1e′⁶ at/cm² ofhydrogen at the geometric center of a first substrate consisting of asilicon board. After assembling with a support also made of a siliconboard, the fracture was initiated at the incipient fracture, during anembrittlement heat treatment at 350° C. for 30 hours.

The fracture wave propagated at a speed of the order of 2.8 km/sec.

It can be noted that the useful layer shown in FIG. 5A has periodicalpatterns of variation in the thickness, which are confined to aperipheral annular area, the inner radius of which is larger than 80% ofthe radius of the useful layer. The central surface area of the usefullayer is free of periodical patterns of variation in thickness.

FIG. 5B shows a view of a useful layer obtained by a simulation of amethod according to the disclosure, when the fracture wave is propagatedat 4 km/second.

This view of the useful layer 3 shows that a very narrow annular areacomprising the periodical patterns of variation in thickness is providedon the periphery.

This last figure is a perfect illustration of the advantage that can betaken from the present disclosure to form a useful layer with enhancedevenness properties.

What is claimed is:
 1. A substrate, comprising: a useful layer affixedto a support substrate; wherein a surface of the useful layer located ona side of the useful layer opposite the support substrate comprises: afirst region comprising a first surface roughness, the first regionlocated proximate to a geometric center of the surface, the first regionoccupying a majority of an area the surface; and a second regioncomprising a second, higher surface roughness, the second region locatedproximate to a periphery of the surface, the second region occupying aminority of the area of the surface; wherein the first region is atleast substantially free of periodical patterns in variations of surfaceheight, and the second region comprises periodical patterns invariations in surface height.
 2. The substrate of claim 1, wherein thefirst region occupies 80% or more of the area of the surface.
 3. Thesubstrate of claim 1, wherein the first region is at least substantiallycircular and the second region is at least substantially annular, whenthe first region and the second region are viewed in a directionperpendicular to the surface.
 4. The substrate of claim 1, wherein thefirst region is configured to emit a first haze, and the second regionis configured to emit a second, higher haze.
 5. The substrate of claim1, wherein the first region is confined within an exclusion area of theuseful layer.
 6. The substrate of claim 1, wherein a shortest distancefrom the periphery of the surface to a boundary between the first regionand the second region is between about 0.5 mm and about 3 mm.
 7. Thesubstrate of claim 1, wherein a material of the useful layer comprisessilicon, germanium, gallium nitride, lithium tantalate, or sapphire. 8.The substrate of claim 1, wherein a material of the support substratecomprises silicon, sapphire, or glass.
 9. The substrate of claim 1,wherein the useful layer is affixed to the support substrate bymolecular adhesion, thermo-compression, electrostatic bonding, or anadhesive material.
 10. A substrate, comprising: a useful layercomprising a semiconductor material affixed to a support substrate;wherein a surface of the useful layer located on a side of the usefullayer opposite the support substrate comprises: a first regionconfigured to emit a first haze, the first region located proximate to ageometric center of the surface, the first region occupying a majorityof an area the surface; and a second region configured to emit a second,higher haze, the second region located proximate to a periphery of thesurface, the second region occupying a minority of the area of thesurface; wherein the first region is at least substantially free ofperiodical patterns in variations of surface height, and the secondregion comprises periodical patterns in variations in surface height.11. The substrate of claim 10, wherein the first region occupies 80% ormore of the area of the surface.
 12. The substrate of claim 10, whereinthe first region is at least substantially circular and the secondregion is at least substantially annular, when the first region and thesecond region are viewed in a direction perpendicular to the surface.13. The substrate of claim 10, wherein the first region comprises afirst surface roughness, and the second region comprises a second,higher surface roughness.
 14. The substrate of claim 10, wherein thefirst region is located within an exclusion area of the useful layer.15. The substrate of claim 10, wherein a shortest distance from theperiphery of the surface to a boundary between the first region and thesecond region is between about 0.5 mm and about 3 mm.
 16. The substrateof claim 10, wherein the semiconductor material of the useful layercomprises silicon, germanium, gallium nitride, or sapphire.
 17. Thesubstrate of claim 10, wherein a material of the support substratecomprises silicon, sapphire, or glass.
 18. The substrate of claim 10,wherein the useful layer is affixed to the support substrate bymolecular adhesion, thermo-compression, electrostatic bonding, or anadhesive material.